Liquid-crystalline medium

ABSTRACT

The invention relates to a liquid-crystalline medium based on a mixture of polar compounds of positive dielectric anisotropy, characterised in that it comprises one or more ester compounds of the formula I 
                         
and one or more compounds of the formula IA
 
                         
in which R 1 , R 2 , ring A, ring B, L 1-6 , Z 1 , Z 2 , X 1  and X 2  are as defined in claim 1.

This application is a Continuation of U.S. application Ser. No.10/359,282, filed Feb. 6, 2003 now abandoned.

The present invention relates to a liquid-crystalline medium, to the usethereof for electro-optical purposes, and to displays containing thismedium.

Liquid-crystals are used principally as dielectrics in display devices,since the optical properties of such substances can be modified by anapplied voltage. Electro-optical devices based on liquid crystals areextremely well known to the person skilled in the art and can be basedon various effects. Examples of such devices are cells having dynamicscattering, DAP (deformation of aligned phases) cells, guest/host cells,TN cells having a twisted nematic structure, STN (supertwisted nematic)cells, SBE (super-birefringence effect) cells and OMI (optical modeinterference) cells. The commonest display devices are based on theSchadt-Helfrich effect and have a twisted nematic structure.

The liquid-crystal materials must have good chemical and thermalstability and good stability to electric fields and electromagneticradiation. Furthermore, the liquid-crystal materials should have lowviscosity and produce short addressing times, low threshold voltages andhigh contrast in the cells.

They should furthermore have a suitable mesophase, for example a nematicor cholesteric mesophase for the above-mentioned cells, at the usualoperating temperatures, i.e. in the broadest possible range above andbelow room temperature. Since liquid crystals are generally used asmixtures of a plurality of components, it is important that thecomponents are readily miscible with one another. Further properties,such as the electrical conductivity, the dielectric anisotropy and theoptical anisotropy, have to satisfy various requirements depending onthe cell type and area of application. For example, materials for cellshaving a twisted nematic structure should have positive dielectricanisotropy and low electrical conductivity.

For example, for matrix liquid-crystal displays with integratednon-linear elements for switching individual pixels (MLC displays),media having large positive dielectric anisotropy, broad nematic phases,relatively low birefringence, very high specific resistance, good UV andtemperature stability and lower vapour pressure are desired.

Matrix liquid-crystal displays of this type are known. Non-linearelements which can be used for individual switching of the individualpixels are, for example, active elements (i.e. transistors). The term“active matrix” is then used, where a distinction can be made betweentwo types:

-   1. MOS (metal oxide semiconductor) or other diodes on a silicon    wafer as substrate.-   2. Thin-film transistors (TFTs) on a glass plate as substrate.

The use of single-crystal silicon as substrate material restricts thedisplay size, since even modular assembly of various part-displaysresults in problems at the joins.

In the case of the more promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A distinction ismade between two technologies: TFTs comprising compound semiconductors,such as, for example, CdSe, or TFTs based on polycrystalline oramorphous silicon. Intensive work is being carried out world-wide on thelatter technology.

The TFT matrix is applied to the inside of one glass plate of thedisplay, while the other glass plate carries the transparentcounterelectrode on its inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fully colour-capabledisplays, in which a mosaic of red, green and blue filters is arrangedin such a way that a filter element is opposite each switchable pixel.

The TFT displays usually operate as TN cells with crossed polarisers intransmission and are illuminated from the back.

The term MLC displays here covers any matrix display with integratednon-linear elements, i.e., besides the active matrix, also displays withpassive elements, such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications(for example pocket TVs) or for high-information displays for computerapplications (laptops) and in automobile or aircraft construction.Besides problems regarding the angle dependence of the contrast and theresponse times, difficulties also arise in MLC displays due toinsufficiently high specific resistance of the liquid-crystal mixtures[TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K.,TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September1984: A 210–288 Matrix LCD Controlled by Double Stage Diode Rings, p.141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Designof Thin Film Transistors for Matrix Addressing of Television LiquidCrystal Displays, p. 145 ff, Paris]. With decreasing resistance, thecontrast of an MLC display deteriorates, and the problem of after-imageelimination may occur. Since the specific resistance of theliquid-crystal mixture generally drops over the life of an MLC displayowing to interaction with the interior surfaces of the display, a high(initial) resistance is very important in order to obtain acceptableservice lives. In particular in the case of low-volt mixtures, it washitherto impossible to achieve very high specific resistance values. Itis furthermore important that the specific resistance exhibits thesmallest possible increase with increasing temperature and after heatingand/or UV exposure. The low-temperature properties of the mixtures fromthe prior art are also particularly disadvantageous. It is demanded thatno crystallisation and/or smectic phases occur, even at lowtemperatures, and the temperature dependence of the viscosity is as lowas possible. The MLC displays from the prior art thus do not meettoday's requirements.

There thus continues to be a great demand for MLC displays having veryhigh specific resistance at the same time as a large working-temperaturerange, short response times even at low temperatures and low thresholdvoltage which do not have these disadvantages, or only do so to areduced extent.

In addition to liquid-crystal displays which use back-lighting, i.e. areoperated transmissively and if desired transflectively, reflectiveliquid-crystal displays are also particularly interesting. Thesereflective liquid-crystal displays use the ambient light for informationdisplay. They thus consume significantly less energy than back-litliquid-crystal displays having a corresponding size and resolution.Since the TN effect is characterised by very good contrast, reflectivedisplays of this type can even be read well in bright ambientconditions. This is already known of simple reflective TN displays, asused, for example, in watches and pocket calculators. However, theprinciple can also be applied to high-quality, higher-resolution activematrix-addressed displays, such as, for example, TFT displays. Here, asalready in the transmissive TFT-TN displays which are generallyconventional, the use of liquid crystals of low birefringence (Δn) isnecessary in order to achieve low optical retardation (d*Δn). This lowoptical retardation results in usually acceptable low viewing-angledependence of the contrast (cf. DE 30 22 818). In reflective displays,the use of liquid crystals of low birefringence is even more importantthan in transmissive displays since the effective layer thicknessthrough which the light passes is approximately twice as large inreflective displays as in transmissive displays having the same layerthickness.

In TN (Schadt-Helfrich) cells, media are desired which facilitate thefollowing advantages in the cells:

-   -   extended nematic phase range (in particular down to low        temperatures)    -   stable on storage, even at low temperatures    -   the ability to switch at extremely low temperatures (outdoor        use, automobile, avionics)    -   increased resistance to UV radiation (longer service life)    -   low optical birefringence (Δn) for reflective displays.

The media available from the prior art do not allow these advantages tobe achieved while simultaneously retaining the other parameters.

In the case of supertwisted (STN) cells, media are desired which enablegreater multiplexability and/or lower threshold voltages and/or broadernematic phase ranges (in particular at low temperatures). To this end, afurther widening of the available parameter latitude (clearing point,smectic-nematic transition or melting point, viscosity, dielectricparameters, elastic parameters) is urgently desired.

The invention has the object of providing media, in particular for MLC,TN or STN displays of this type, which do not have the above-mentioneddisadvantages or only do so to a reduced extent, and preferablysimultaneously have very low threshold voltages and at the same timehigh values for the voltage holding ratio (VHR).

It has now been found that this object can be achieved if mediaaccording to the invention are used in displays.

The invention thus relates to a liquid-crystalline medium based on amixture of polar compounds of positive dielectric anisotropy,characterised in that it comprises one or more ester compounds of theformula I

and one or more compounds of the formula IA

in which the individual radicals have the following meanings:

-   R¹ and R² are each, independently of one another, H, a halogenated    unsubstituted alkyl radical having from 1 to 15 carbon atoms, where    one or more CH₂ groups in these radicals may also be replaced, in    each case independently of one another, by —C≡C—, —CH═CH—, —O—,    —CO—O— or —O—CO— in such a way that O atoms are not linked directly    to one another,-   X¹ and X² are each, independently of one another, F, Cl, CN, SF₅,    SCN, NCS, a halogenated alkyl radical, a halogenated alkenyl    radical, a halogenated alkoxy radical or a halogenated alkenyloxy    radical, each having up to 6 carbon atoms,-   Z¹ and Z² are each, independently of one another, —CF₂O—, —OCF₂— or    a single bond, where Z¹≠Z²,

are each, independently of one another,

-   L¹⁻⁶ are each, independently of one another, H or F.

The compounds of the formulae I and IA have a broad range ofapplications. Depending on the choice of substituents, these compoundscan serve as base materials of which liquid-crystalline media arepredominantly composed; however, it is also possible to add compounds ofthe formulae I and IA to liquid-crystalline base materials from otherclasses of compound in order, for example, to modify the dielectricand/or optical anisotropy of a dielectric of this type and/or in orderto optimise its threshold voltage and/or its viscosity.

In the pure state, the compounds of the formulae I and IA are colourlessand form liquid-crystalline mesophases in a temperature range which isfavourably located for electro-optical use. They are stable chemically,thermally and to light.

If R¹ and/or R² are an alkyl radical and/or an alkoxy radical, this maybe straight-chain or branched. It is preferably straight-chain, has 2,3, 4, 5, 6 or 7 carbon atoms and accordingly is preferably ethyl,propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy,hexyloxy or heptyloxy, furthermore methyl, octyl, nonyl, decyl, undecyl,dodecyl, tridecyl, tetradecyl, pentadecyl, methoxy, octyloxy, nonyloxy,decyloxy, undecyloxy, dodecyloxy, tridecyloxy or tetradecyloxy.

Oxaalkyl is preferably straight-chain 2-oxapropyl (=methoxymethyl),2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl,2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl,or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.

If R¹ and/or R² are an alkyl radical in which one CH₂ group has beenreplaced by —CH═CH—, this may be straight-chain or branched. It ispreferably straight-chain and has 2 to 10 carbon atoms. Accordingly, itis in particular vinyl, prop-1- or prop-2-enyl, but-1-, -2- or -3-enyl,pent-1-, -2-, -3- or -4-enyl, hex-1-, -2-, -3-, 4- or -5-enyl, hept-1-,-2-, -3-, -4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, -5-, -6- or-7-enyl, non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, or dec-1-,-2-, -3-, -4-, -5-, -6-, -7-, -8- or -9-enyl.

If R¹ and/or R² are an alkyl radical in which one CH₂ group has beenreplaced by —O— and one has been replaced by —CO—, these are preferablyadjacent. These thus contain an acyloxy group —CO—O— or an oxycarbonylgroup —O—CO. These are preferably straight-chain and have 2 to 6 carbonatoms. Accordingly, they are in particular acetoxy, propionyloxy,butyryloxy, pentanoyloxy, hexanoyloxy, acetoxymethyl,propionyloxymethyl, butyryloxymethyl, pentanoyloxymethyl,2-acetoxyethyl, 2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetoxypropyl,3-propionyloxypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl,propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl,ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl,2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl,2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)propyl,3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.

If R¹ and/or R² are an alkyl radical in which one CH₂ group has beenreplaced by unsubstituted or substituted —CH═CH— and an adjacent CH₂group has been replaced by CO or CO—O or O—CO, this may bestraight-chain or branched. It is preferably straight-chain and has 4 to12 carbon atoms. Accordingly, it is in particular acryloyloxymethyl,2-acryloyloxyethyl, 3-acryloyloxypropyl, 4-acryloyloxybutyl,5-acryloyloxypentyl, 6-acryloyloxyhexyl, 7-acryloyloxyheptyl,8-acryloyloxyoctyl, 9-acryloyloxynonyl, 10-acryloyloxydecyl,methacryoyloxymethyl, 2-methacryloyloxyethyl, 3-methacryloyloxypropyl,4-methacryloyloxybutyl, 5-methacryloyloxypentyl, 6-methacryloyloxyhexyl,7-methacryloyloxyheptyl, 8-methacryloyloxyoctyl or9-methacryloyloxynonyl.

If R¹ and/or R² are an alkyl or alkenyl radical which is monosubstitutedby CN or CF₃, this radical is preferably straight-chain. Thesubstitution by CN or CF₃ is in any desired position.

If R¹ and/or R² are an alkyl or alkenyl radical which is at leastmonosubstituted by halogen, this radical is preferably straight-chain,and halogen is preferably F or Cl. In the case of polysubstitution,halogen is preferably F. The resultant radicals also includeperfluorinated radicals. In the case of monosubstitution, the fluorineor chlorine substituent may be in any desired position, but ispreferably in the co-position.

Compounds containing branched wing groups R¹ and/or R² may occasionallybe of importance owing to better solubility in the conventionalliquid-crystalline base materials, but in particular as chiral dopantsif they are optically active. Smectic compounds of this type aresuitable as components of ferroelectric materials.

Branched groups of this type generally contain not more than one chainbranch. Preferred branched radicals R¹ and/or R² are isopropyl, 2-butyl(=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl(=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl,2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy,3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexyloxy,1-methylhexyloxy and 1-methylheptyloxy.

If R¹ and/or R² are an alkyl radical in which two or more CH₂ groupshave been replaced by —O— and/or —CO—O—, this may be straight-chain orbranched. It is preferably branched and has from 3 to 12 carbon atoms.Accordingly, it is in particular biscarboxymethyl, 2,2-biscarboxyethyl,3,3-biscarboxypropyl, 4,4-biscarboxybutyl, 5,5-biscarboxypentyl,6,6-biscarboxyhexyl, 7,7-biscarboxyheptyl, 8,8-biscarboxyoctyl,9,9-biscarboxynonyl, 10,10-biscarboxydecyl, bis(methoxycarbonyl)methyl,2,2-bis(methoxycarbonyl)ethyl, 3,3-bis(methoxycarbonyl)propyl,4,4-bis(methoxycarbonyl)butyl, 5,5-bis(methoxycarbonyl)pentyl,6,6-bis(methocycarbonyl)hexyl, 7,7-bis(methoxycarbonyl)heptyl,8,8-bis(methoxycarbonyl)octyl, bis(ethoxycarbonyl)methyl,2,2-bis(ethoxycarbonyl)ethyl, 3,3-bis(ethoxycarbonyl)propyl,4,4-bis(ethoxycarbonyl)butyl or 5,5-bis(ethoxycarbonyl)hexyl.

The compounds of the formulae I and IA are prepared by methods known perse, as described in the literature (for example in the standard works,such as Houben-Weyl, Methoden der organischen Chemie [Methods of OrganicChemistry], Georg-Thieme-Verlag, Stuttgart), to be precise underreaction conditions which are known and suitable for the said reactions.Use can also be made here of variants which are known per se, but arenot mentioned here in greater detail. The compounds of the formula IAare known, for example, from DE-A40 06 921, EP-A 1 046 693 A1 and EP-A 1046 694 A1.

The invention also relates to electro-optical displays (in particularSTN or MLC displays having two plane-parallel outer plates, which,together with a frame, form a cell, integrated non-linear elements forswitching individual pixels on the outer plates, and a nematicliquid-crystal mixture of positive dielectric anisotropy and highspecific resistance which is located in the cell) which contain media ofthis type, and to the use of these media for electro-optical purposes.

The liquid-crystal mixtures according to the invention enable asignificant widening of the available parameter latitude.

The achievable combinations of clearing point, viscosity at lowtemperature, thermal and UV stability and dielectric anisotropy are farsuperior to previous materials from the prior art.

Compared with the mixtures disclosed in EP 1 046 693 A1, the mixturesaccording to the invention have a higher clearing point, low γ₁ valuesand very high values for the VHR at 100° C. The mixtures according tothe invention are preferably suitable as TN-TFT mixtures for notebook PCapplications with 3.3 and 2.5 V drivers.

The requirement for a high clearing point, a nematic phase at lowtemperature and a high Δ∈ has hitherto only been achieved to aninadequate extent. Although mixtures such as, for example, ZLI-3119 havea comparable clearing point and comparably favourable viscosities, theyhave, however, a Δ∈ of only +3.

Other mixture systems have comparable viscosities and Δ∈ values, butonly have clearing points in the region of 60° C.

The liquid-crystal mixtures according to the invention, while retainingthe nematic phase down to −20° C. and preferably down to −30° C.,particularly preferably down to −40° C., enable clearing points above60° C., preferably above 65° C., particularly preferably above 70° C.,simultaneously dielectric anisotropy values Δ∈ of >6, preferably >8, anda high value for the specific resistance to be achieved, enablingexcellent STN and MLC displays to be obtained. In particular, themixtures are characterised by low operating voltages. The TN thresholdsare below 2.0 V, preferably below 1.5 V, particularly preferably <1.3 V.

It goes without saying that, through a suitable choice of the componentsof the mixtures according to the invention, it is also possible forhigher clearing points (for example above 110°) to be achieved at ahigher threshold voltage or lower clearing points to be achieved atlower threshold voltages with retention of the other advantageousproperties. At viscosities correspondingly increased only slightly, itis likewise possible to obtain mixtures having greater Δ∈ and thus lowerthresholds. The MLC displays according to the invention preferablyoperate at the first Gooch and Tarry transmission minimum [C. H. Goochand H. A. Tarry, Electron. Lett. 10, 2–4, 1974; C. H. Gooch and H. A.Tarry, Appl. Phys., Vol. 8, 1575–1584, 1975] are used, where, besidesparticularly favourable electro-optical properties, such as, forexample, high steepness of the characteristic line and low angledependence of the contrast (German Patent 30 22 818), a lower dielectricanisotropy is sufficient at the same threshold voltage as in ananalogous display at the second minimum. This enables significantlyhigher specific resistances to be achieved using the mixtures accordingto the invention at the first minimum than in the case of mixturescomprising cyano compounds. Through a suitable choice of the individualcomponents and their proportions by weight, the person skilled in theart is able to set the birefringence necessary for a pre-specified layerthickness of the MLC display using simple routine methods.

The flow viscosity ν₂₀ at 20° C. is preferably <60 mm²·s⁻¹, particularlypreferably <50 mm²·s⁻¹. The rotational viscosity γ₁ at 20° C. of themixtures according to the invention is preferably <160 mPa·s,particularly preferably <150 mPa·s. The nematic phase range ispreferably at least 90°, in particular at least 100°. This rangepreferably extends at least from −20° to +80°.

A short response time is desired in liquid-crystal displays. Thisapplies in particular to displays which are capable of videoreproduction. For displays of this type, response times (sum:t_(on)+t_(off)) of at most 16 ms are required. The upper limit of theresponse time is determined by the image refresh frequency.

Measurements of the voltage holding ratio (HR) [S. Matsumoto et al.,Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference,San Francisco, June 1984, p. 304 (1984); G. Weber et al., LiquidCrystals 5, 1381 (1989)] have shown that mixtures according to theinvention comprising compounds of the formulae I and IA exhibit asignificantly smaller decrease in the HR with increasing temperaturethan, for example, analogous mixtures comprising cyanophenylcyclohexanesof the formula

or esters of the formula

instead of the compounds of the formula IA.

The UV stability of the mixtures according to the invention is alsoconsiderably better, i.e. they exhibit a significantly smaller decreasein the HR on exposure to UV.

Formula I preferably covers the following compounds:

in which R¹ is as defined in claim 1. R¹ is preferably H, CH₃, C₂H₅,n-C₃H₇, n-C₄H₉, n-C₅H₁₁, n-C₆H₁₃, CH₂═CH, CH₃CH═CH or 3-alkenyl.

Preference is given to media according to the invention which compriseat least one compound of the formulae I-1, I-2, I-8 and/or I-11,particularly preferably in each case at least one compound of theformula I-2.

Particularly preferred compounds of the formula IA are compounds of theformulae IA-1 to IA-25:

in which R² is as defined above.

Of these preferred compounds, particular preference is given to those ofthe formulae IA-1, IA-2, IA-3, IA-4 and IA-15, in particular those ofthe formulae IA-1 and IA-2.

R² in the compounds of the formulae IA and IA1 to IA-24 is preferably H,straight-chain alkyl having from 1 to 7 carbon atoms, in particular CH₃,C₂H₅, n-C₃H₇, N—C₄H₉, n-C₅H, n-C₆H₁₃, n-C₇H₁₅, furthermore 1E- or3-alkenyl, in particular CH₂═CH, CH₃CH═CH, CH₂═CHCH₂CH₂ orCH₃CH═CH—CH₂CH₂.

The compounds of the formula IA have been disclosed, for example, in EP0 334 911 B1 and WO 01/25370.

Preferred embodiments are indicated below:

-   -   The medium comprises one, two or more compounds selected from        the group consisting of the formulae IA-1 to IA-24;    -   The medium preferably comprises in each case one or more,        preferably two or three, compounds (homologues) of the formulae        I-2 and IA-15;    -   The medium preferably comprises in each case one or more,        preferably two or three, compounds (homologs) of the formulae        I-2 and IA-3;    -   The medium additionally comprises one or more compounds selected        from the group consisting of the general formulae II to VI:

-   -    in which the individual radicals are as defined below:    -   R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having up        to 9 carbon atoms,    -   X⁰ is F, Cl, halogenated alkyl, alkenyl, alkenyloxy or alkoxy        having up to 6 carbon atoms,    -   Z⁰ is —C₂F₄—, —CF═CF—, —C₂H₄—, —(CH₂)₄—, —CF₂O—, —OCF₂—, —OCH₂—        or —CH₂O—,    -   Y¹ to Y⁴ are each, independently of one another, H or F,    -   r is 0 or 1.

The compound of the formula IV is preferably

-   -   The medium additionally comprises one or more compounds selected        from the group consisting of the general formulae VII to XIII:

-   -    in which R⁰, X⁰, Y¹ and Y² are each, independently of one        another, as defined in claim 4. Y³ is H or F. X⁰ is preferably        F, Cl, CF₃, OCF₃ or OCHF₂. R⁰ is preferably alkyl, oxaalkyl,        fluoroalkyl or alkenyl, each having up to 6 carbon atoms.    -   The medium additionally comprises one or more ester compounds of        the formulae Ea to Ed

-   -    in which R⁰ is as defined in claim 4;    -   The proportion of the compounds of the formulae Ea to Ed is        preferably 10–30% by weight, in particular 15–25% by weight;    -   The proportion of compounds of the formulae IA and I to VI        together in the mixture as a whole is at least 50% by weight;    -   The proportion of compounds of the formula I in the mixture as a        whole is from 5 to 40% by weight, particularly preferably from        10 to 30% by weight;    -   The proportion of compounds of the formula IA in the mixture as        a whole is from 5 to 40% by weight, particularly preferably from        10 to 30% by weight;    -   The proportion of compounds of the formulae II to VI in the        mixture as a whole is from 30 to 80% by weight;

-   -   The medium comprises compounds of the formula II, III, IV, V or        VI;    -   R⁰ is straight-chain alkyl or alkenyl having from 2 to 7 carbon        atoms;    -   The medium essentially consists of compounds of the formulae IA,        I to VI and XIII;    -   The medium comprises further compounds, preferably selected from        the following group consisting of the general formulae XIV to        XVII:

-   -    in which R⁰ and X⁰ are as defined above, and the 1,4-phenylene        rings may be substituted by CN, chlorine or fluorine. The        1,4-phenylene rings are preferably monosubstituted or        polysubstituted by fluorine atoms.    -   The medium additionally comprises one or more compounds of the        formula XVIII

-   -    in which R⁰, X⁰, Y¹ and Y² are as defined above.    -   The medium additionally comprises one, two, three or more,        preferably two or three, compounds of the formulae

-   -    in which “alkyl” and “alkyl*” are as defined below.    -   The proportion of the compounds of the formulae O1 and/or O2 in        the mixtures according to the invention is preferably 5–10% by        weight.    -   The medium preferably comprises 5–35% by weight of compound IVa.    -   The medium preferably comprises one, two or three compounds of        the formula IVa in which X⁰ is F or OCF₃.    -   The medium preferably comprises one or more compounds of the        formulae IIa to IIg

-   -    in which R⁰ is as defined above. In the compounds of the        formulae IIa–IIg, R⁰ is preferably H, methyl, ethyl, n-propyl,        n-butyl or n-pentyl, furthermore n-hexyl or n-heptyl.

The (I+IA):(II+III+IV+V+VI) weight ratio is preferably from 1:10 to10:1.

-   -   The medium essentially consists of compounds selected from the        group consisting of the general formulae IA and I to XIII.    -   The proportion of the compounds of the formulae IVb and/or IVc        in which X⁰ is fluorine and R⁰ is C₂H₅, n-C₃H₇, n-C₄H₅ or        n-C₅H₁₁ in the mixture as a whole is from 2 to 20% by weight, in        particular from 2 to 15% by weight;    -   The medium preferably comprises one, two or three, furthermore        four, homologs of the compounds selected from the group        consisting of H1 to H18 (n=1–12):

-   -   The medium comprises further compounds, preferably selected from        the following group consisting of the general formulae XVI to        XIX:

-   -    in which R⁰ and X⁰ are as defined above, and the 1,4-phenylene        rings may be substituted by CN, chlorine or fluorine. The        1,4-phenylene rings are preferably monosubstituted or        polysubstituted by fluorine atoms.    -   The medium comprises further compounds, preferably selected from        the following group consisting of the formulae RI to RVIII

-   -    in which        -   R⁰ is n-alkyl, oxaalkyl, fluoroalkyl, alkenyloxy or alkenyl,            each having up to 9 carbon atoms,        -   Y¹ is H or F,        -   alkyl and alkyl* are each, independently of one another, a            straight-chain or branched alkyl radical having 1–9 carbon            atoms,        -   alkenyl and alkenyl* are each, independently of one another,            a straight-chain or branched alkenyl radical having up to 9            carbon atoms.    -   The medium preferably comprises one or more compounds of the        formulae

-   -    in which    -   n and m are each 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12. n and        m are preferably 1, 2, 3, 4, 5 or 6.    -   Medium additionally comprising one, two or more compounds having        fused rings, of the formulae AN1 to AN11:

-   -    in which R⁰ is as defined above.

It has been found that even a relatively small proportion of compoundsof the formulae I and IA mixed with conventional liquid-crystalmaterials, but in particular with one or more compounds of the formulaeI, III, IV, V and/or VI, results in a lowering of the threshold voltageand in high values for the VHR (100° C.), with broad nematic phases withlow smectic-nematic transition temperatures being observed at the sametime, improving the shelf life. Preference is given, in particular, tomixtures which, besides one or more compounds of the formulae I and IA,comprise one or more compounds of the formula IV, in particularcompounds of the formula IVa in which X⁰ is F or OCF₃. The compounds ofthe formulae IA and I to VI are colourless, stable and readily misciblewith one another and with other liquid-crystalline materials.

The term “alkyl” or “alkyl*” covers straight-chain and branched alkylgroups having 1–7 carbon atoms, in particular the straight-chain groupsmethyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groups having2–5 carbon atoms are generally preferred.

The term “alkenyl” covers straight-chain and branched alkenyl groupshaving 2–7 carbon atoms, in particular the straight-chain groups.Preferred alkenyl groups are C₂–C₇-1E-alkenyl, C₄–C₇-3E-alkenyl,C₅–C₇-4-alkenyl, C₆–C₇-5-alkenyl and C₇-6-alkenyl, in particularC₂–C₇-1E-alkenyl, C₄–C₇-3E-alkenyl and C₅–C₇-4-alkenyl. Examples ofparticularly preferred alkenyl groups are vinyl, 1E-propenyl,1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl,3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl,4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groupshaving up to 5 carbon atoms are generally preferred.

The term “fluoroalkyl” preferably covers straight-chain groups having aterminal fluorine, i.e. fluoromethyl, 2-fluoroethyl, 3-fluoropropyl,4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl.However, other positions of the fluorine are not excluded.

The term “oxaalkyl” preferably covers straight-chain radicals of theformula C_(n)H_(2n+1)—O—(CH₂)_(m), in which n and m are each,independently of one another, from 1 to 6. Preferably, n=1 and m is from1 to 6.

Through a suitable choice of the meanings of R⁰ and X⁰, the addressingtimes, the threshold voltage, the steepness of the transmissioncharacteristic lines, etc., can be modified in the desired manner. Forexample, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxyradicals and the like generally result in shorter addressing times,improved nematic tendencies and a higher ratio of the elastic constantsk₃₃ (bend) and k₁₁ (splay) compared with alkyl or alkoxy radicals.4-alkenyl radicals, 3-alkenyl radicals and the like generally give lowerthreshold voltages and smaller values of k₃₃/k₁₁ compared with alkyl andalkoxy radicals.

A —CH₂CH₂— group generally results in higher values of k₃₃/k₁₁ comparedwith a single covalent bond. Higher values of k₃₃/k₁₁ facilitate, forexample, flatter transmission characteristic lines in TN cells with a90° twist (in order to achieve grey shades) and steeper transmissioncharacteristic lines in STN, SBE and OMI cells (greatermultiplexability), and vice versa.

The optimum mixing ratio of the compounds of the formulae I, IA andII+III+IV+V+VI depends substantially on the desired properties, on thechoice of the components of the formulae I, IA, II, III, IV, V and/orVI, and on the choice of any other components that may be present.Suitable mixing ratios within the range given above can easily bedetermined from case to case.

The total amount of compounds of the formulae IA and I to XIII in themixtures according to the invention is not crucial. The mixtures cantherefore comprise one or more further components for the purposes ofoptimisation of various properties. However, the observed effect on theaddressing times and the threshold voltage is generally greater, thehigher the total concentration of compounds of the formulae IA and I toXIII.

In a particularly preferred embodiment, the media according to theinvention comprise compounds of the formulae II to VI (preferably II,III and/or IV, in particular IVa) in which X⁰ is F, OCF₃, OCHF₂, F,OCH═CF₂, OCF═CF₂ or OCF₂—CF₂H. A favourable synergistic effect with thecompounds of the formulae I and IA results in particularly advantageousproperties. In particular, mixtures comprising compounds of the formulaI, IA and of the formula IVa are distinguished by their low thresholdvoltages.

The individual compounds of the formulae IA and I to XVIII and theirsub-formulae which can be used in the media according to the inventionare either known or can be prepared analogously to the known compounds.

The construction of the MLC display according to the invention frompolarisers, electrode base plates and surface-treated electrodescorresponds to the conventional construction for displays of this type.The term conventional construction is broadly drawn here and also coversall derivatives and modifications of the MLC display, in particularincluding matrix display elements based on poly-Si TFT or MIM.

A significant difference between the displays according to the inventionand the conventional displays based on the twisted nematic cellconsists, however, in the choice of the liquid-crystal parameters of theliquid-crystal layer.

The liquid-crystal mixtures which can be used in accordance with theinvention are prepared in a manner conventional per se. In general, thedesired amount of the components used in the lesser amount is dissolvedin the components making up the principal constituent, advantageously atelevated temperature. It is also possible to mix solutions of thecomponents in an organic solvent, for example in acetone, chloroform ormethanol, and to remove the solvent again, for example by distillation,after thorough mixing.

The dielectrics may also comprise further additives known to the personskilled in the art and described in the literature. For example, 0–15%of pleochroic dyes, antioxidants, UV absorbers and/or chiral dopants canbe added.

C denotes a crystalline phase, S a smectic phase, S_(C) a smectic Cphase, N a nematic phase and I the isotropic phase.

V₁₀ denotes the voltage for 10% transmission (viewing angleperpendicular to the plate surface). t_(on) denotes the switch-on timeand t_(off) the switch-off time at an operating voltage corresponding to2.0 times the value of V₁₀. Δ n denotes the optical anisotropy. Asdenotes the dielectric anisotropy (Δ∈=∈_(∥)−∈_(⊥), where ∈_(∥) denotesthe dielectric constant parallel to the longitudinal molecular axes and∈_(⊥)denotes the dielectric constant perpendicular thereto). Theelectro-optical data were measured in a TN cell at the 1st minimum (i.e.at a d·Δn value of 0.5 μm) at 20° C., unless expressly stated otherwise.The optical data were measured at 20° C., unless expressly statedotherwise.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by means of acronyms, thetransformation into chemical formulae taking place in accordance withTables A and B below. All radicals C_(n)H_(2n+1) and C_(m)H_(2m+1) arestraight-chain alkyl radicals having n and m carbon atoms respectively;n and m are integers and are preferably 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 11 or 12. The coding in Table B is self-evident. In Table A, onlythe acronym for the parent structure is indicated. In individual cases,the acronym for the parent structure is followed, separated by a dash,by a code for the substituents R^(1*), R^(2*), L^(1*), L^(2*) andL^(3*):

Code for R¹*, R²*, L¹*, L²*, L³* R¹* R²* L¹* L²* L³* nm C_(n)H_(2n+1)C_(m)H_(2m+1) H H H nOm OC_(n)H_(2n+1) C_(m)H_(2m+1) H H H nO.mC_(n)H_(2n+1) OC_(m)H_(2m+1) H H H n C_(n)H_(2n+1) CN H H H nN.FC_(n)H_(2n+1) CN H H F nN.F.F C_(n)H_(2n+1) CN H F F nF C_(n)H_(2n+1) FH H H nOF OC_(n)H_(2n+1) F H H H nF.F C_(n)H_(2n+1) F H H F nmFC_(n)H_(2n+1) C_(m)H_(2m+1) F H H nOCF₃ C_(n)H_(2n+1) OCF₃ H H H nOCF₃.FC_(n)H_(2n+1) OCF₃ F H H n-Vm C_(n)H_(2n+1) —CH═CH—C_(m)H_(2m+1) H H HnV-Vm C_(n)H_(2n+1)— —CH═CH—C_(m)H_(2m+1) H H H

Preferred mixture components are given in Tables A and B.

TABLE A

PYP

PYRP

BCH

CBC

CCH

CCP

CPTP

CEPTP

ECCP

CECP

EPCH

PCH

PTP

BECH

EBCH

CPC

B

FET-n-F

CGG

CGU

CFU

PGU

TABLE B

BCH-n.Fm

CFU-n-F

CBC-nmF

ECCP-nm

CCZU-n-F

T-nFm

CGU-n-F

CDU-n-F

DCU-n-F

CGG-n-F

CPZG-n-OT

CC-nV-Vm

CCP-Vn-m

CCG-V-F

CCP-nV-m

CC-n-V

CCQU-n-F

CC-n-V1

CCQG-n-F

CQCU-n-F

Dec-U-n-F

CWCU-n-F

CWCG-n-F

CCOC-n-m

CPTU-n-F

GPTU-n-F

PQU-n-F

PUQU-n-F

PGU-n-F

CGZP-n-OT

CCGU-n-F

CCQG-n-F

CUQU-n-F

PCQU-n-F

CUQU-n-F

CGUQU-n-F

CCPU-n-F

CECU-n-F

CCEU-n-F

CPEU-n-F

CPUQU-n-F

Particular preference is given to liquid-crystalline mixtures which,besides the compounds of the formulae I and IA, comprise at least one,two, three or four compounds from Table B.

TABLE C Table C shows possible dopants which are generally added to themixtures according to the invention.

C 15

CB 15

CM 21

R/S-811

CM 44

CM 45

CM 47

R/S-1011

R/S-3011

CN

R/S-2011

TABLE D Stabilisers which can be added, for example, to the mixturesaccording to the invention are mentioned below.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The preceding preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing and in the examples, all temperatures are set forthuncorrected in degrees Celsius and, all parts and percentages are byweight, unless otherwise indicated.

The entire disclosures of all applications, patents and publications,cited herein and of corresponding German application No. 10204790.1,filed Feb. 6, 2002 is incorporated by reference herein.

m.p. denotes melting point, clap. clearing point. Furthermore,C=crystalline state, N=nematic phase, S=smectic phase and I=isotropicphase. The data between these symbols represent the transitiontemperatures. Δn denotes optical anisotropy (589 nm, 20° C.), Δ∈ thedielectric anisotropy (1 kHz, 20° C.), and the flow viscosity ν₂₀(mm²/sec) was determined at 20° C. The rotational viscosity γ₁ (mPa·s)was likewise determined at 20° C.

EXAMPLE M1

PUQU-3-F 7.00% Clearing point [° C.]: +70 BCH-3F.F.F 5.50% Δn [589 nm;20° C.]: +0.0932 CUQU-3-F 5.00% Δε [1 kHz; 20° C.]: 13.5 CCP-2F.F 12.00%VHR (100° C.): 94.5 CCP-3F.F.F 10.00% γ₁ [mPa · s; 20° C.]: 159 CECG-3-F10.00% V_(10,0,20): 0.94 CCZU-3-F 13.00% CCZU-4-F 5.00% CDU-3-F 3.00%DCU-4-F 6.00% CGUQU-2-F 8.50% CGUQU-3-F 4.00% CGZP-2-OT 6.00% CGZP-3-OT5.00%

EXAMPLE M2

CC-3-V 16.00% S→N [° C.]: −40.0 CCZU-2-F 4.00% Clearing point [° C.]:+79.0 CCZU-3-F 15.00% Δn [589 nm; 20° C.]: +0.0883 CGZP-2-OT 10.00% VHR(100° C.): 94.6 CDU-2-F 7.00% γ₁ [mPa · s; 20° C.]: 127 CDU-3-F 9.00%V_(10,0,20): 1.04 CDU-5-F 9.00% CCH-35 5.00% CGU-2-F 3.00% CC-3-V1 2.00%CPUQU-3-F 10.00% CPUQU-2-F 10.00%

EXAMPLE M3

CC-3-V 19.00% S→N [° C.]: −40.0 CCH-35 3.00% Clearing point [° C.]:+79.0 CCZU-2-F 4.00% Δn [589 nm; 20° C.]: +0.0887 CCZU-3-F 14.00% γ₁[mPa · s; 20° C.]: 132 CGZP-2-OT 10.00% V_(10,0,20): 0.99 CGZP-3-OT8.00% VHR (100° C.): 94.5 CDU-2-F 9.00% γ₁ [mPa · s; 20° C.]: 159CDU-3-F 9.00% V_(10,0,20): 0.94 CDU-5-F 4.00% CGUQU-2-F 10.00% CGUQU-3-F10.00%

COMPARATIVE EXAMPLE (EXAMPLE FROM P. 37 of EP 1 046 693)

PUQU-3-F 4.0% Clearing point [° C.]: +70.1 BCH-3F.F.F 21.0% Δn [589 nm;20° C.]: +0.0926 CCPU-3-F 4.0% Δε [1 kHz; 20° C.]: 12.0 CUQU-3-F 4.0%VHR (100° C.): 90.8 CCP-2F.F 7.0% γ₁ [mPa · s; 20° C.]: 161 CCP-3F.F7.0% V_(10,0,20): 1.01 CCP-3F.F.F 8.0% CECU-3-F 10.0% CCEU-3-F 10.0%CCEU-4-F 3.0% CPEU-2-F 2.0% CPEU-3-F 3.0% CDU-3-F 3.0% DCU-4-F 7.0%DCU-5-F 7.0%

EXAMPLE M4

CC-3-V 16.00% CCH-35 4.00% CCP-1F.F.F 11.00% CCP-2F.F.F 9.00% CCP-3F.F.F5.00% CCP-20CF₃.F 11.00% CCZU-2-F 4.00% CCZU-3-F 15.00% CCZU-5-F 4.00%CGZP-2-OT 10.00% CPUQU-2-F 2.00% CPUQU-3-F 7.00% CCOC-3-3 2.00%

EXAMPLE M5

CC-3-V1 3.00% CCH-35 4.00% CC-5-V 12.00% CCH-3CF₃ 4.00% CCP-1F.F.F11.00% CCP-2F.F.F 9.00% CCP-20CF₃ 5.00% CCP-20CF₃.F 6.00% CCZU-2-F 4.00%CCZU-3-F 15.00% CCZU-5-F 4.00% CGZP-2-OT 10.00% CGZP-3-OT 4.00%CPUQU-2-F 7.00% CCOC-3-3 2.00%

EXAMPLE M6

CDU-2-F 9.00% CDU-3-F 9.00% PGU-2-F 9.00% PGU-3-F 6.00% CGZP-2-OT 10.00%CCZU-2-F 4.00% CCZU-3-F 15.00% CCZU-5-F 2.00% CC-5-V 13.00% CC-3-V111.00% CCP-20CF₃ 9.00% CPUQU-2-F 3.00%

EXAMPLE M7

CC-3-V 15.00% S→N [° C.]: −40.0 CCZU-2-F 4.00% Clearing point [° C.]:+80.0 CCZU-3-F 14.00% Δn [589 nm; 20° C.]: +0.0899 CGZP-2-OT 10.00% VHR(100° C.): 92 CDU-2-F 7.00% γ₁ [mPa · s; 20° C.]: 119 CDU-3-F 8.00% HTP[20° C.; μm]: 0.50 CDU-5-F 7.00% Twist [°]: 90 PGU-2-F 1.00% V₁₀ [V]:1.07 CCH-35 4.00% CGU-2-F 2.50% CC-3-V1 7.50% CPUQU-3-F 10.00% CPUQU-2-F10.00%

EXAMPLE M8

CCP-20CF₃ 3.00% S→N [° C.]: −40.0 CCP-30CF₃ 7.00% Clearing point [° C.]:+83.5 CCP-40CF₃ 3.00% Δn [589 nm; 20° C.]: +0.0805 CCZU-2-F 4.00% VHR(100° C.): 98.6 CCZU-3-F 13.00% γ₁ [mPa · s; 20° C.]: 89 CGZP-2-OT 8.00%HTP [20° C.; μm]: 0.50 CDU-2-F 9.00% Twist [°]: 90 CDU-3-F 6.00% V₁₀[V]: 1.30 CC-3-V1 12.00% CC-3-V 18.00% CCH-35 4.00% CGUQU-2-F 10.00%CGUQU-3-F 3.00%

EXAMPLE M9

CC-3-V 18.00% S→N [° C.]: −40.0 CCH-35 4.00% Clearing point [° C.]:+80.0 CCZU-2-F 3.00% Δn [589 nm; 20° C.]: +0.0890 CCZU-3-F 15.00% γ₁[mPa · s; 20° C.]: 135 CGZP-2-OT 10.00% HTP [20° C.; μm]: 0.50 CGZP-3-OT7.50% Twist [°]: 90 CGU-2-F 0.50% V₁₀ [V]: 1.00 CDU-2-F 8.00% CDU-3-F8.00% CDU-5-F 6.00% CGUQU-2-F 10.00% CGUQU-3-F 10.00%

EXAMPLE M10

CCP-20CF₃ 7.00% S→N [° C.]: −40.0 CCP-30CF₃ 4.00% Clearing point [° C.]:+80.5 CCP-2F.F.F 3.00% Δn [589 nm; 20° C.]: +0.0800 CCZU-2-F 4.00% γ₁[mPa · s; 20° C.]: 90 CCZU-3-F 13.00% HTP [20° C.; μm]: 0.50 CGZP-2-OT6.00% Twist [°]: 90 CDU-2-F 7.00% V₁₀ [V]: 1.28 CDU-3-F 7.00% CC-3-V111.00% CC-3-V 18.00% CCH-35 5.00% CGUQU-2-F 10.00% CGUQU-3-F 5.00%

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A liquid-crystalline medium based on a mixture of polar compounds ofpositive dielectric anisotropy, comprising at least one ester compoundof formula I

and at least one compound of formula IA

wherein: R¹ and R² are each, independently H, a halogenatedunsubstituted alkyl radical having from 1 to 15 carbon atoms, where oneor more CH₂ groups in these radicals is optionally independentlyreplaced by —C≡C—, —CH═CH—, —O—, —CO—O— or —O—CO— in such a way that Oatoms are not linked directly to one another, X¹ and X² are each,independently F, Cl, CN, SF₅, SCN, NCS, a halogenated alkyl radical, ahalogenated alkenyl radical, a halogenated alkoxy radical or ahalogenated alkenyloxy radical, each having up to 6 carbon atoms, Z¹ andZ² are each, independently —CF₂O—, —OCF₂— or a single bond, where Z¹≠Z²,

and are each, independently

L³ is F, and L¹, L² and L⁴ to L⁶ are each, independently H or F.
 2. Theliquid-crystalline medium according to claim 1, comprising at least onecompound of formulae IA1–IA25:


3. The liquid-crystalline medium according to claim 1, comprising atleast one compound of formulae I-1 to I-5


4. The liquid-crystalline medium according to claim 1, additionallycomprising at least one compound of the formulae II, III, IV, V or VI:

wherein: R⁰ is H, n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each havingup to 9 carbon atoms, X⁰ is F, Cl, halogenated alkyl, alkenyl or alkoxyhaving up to 6 carbon atoms, Z⁰ is —C₂F₄—, —CF═CF—, —C₂H₄—, —(CH₂)₄—,—CF₂O—, —OCF₂—, —OCH₂— or —CH₂O—, Y¹ and Y² are each, independently H orF, and r is 0 or
 1. 5. The medium according to claim 4, having aproportion of compounds of formulae IA and I to VI together in themixture as a whole of at least 50% by weight.
 6. The medium according toclaim 1, additionally comprising at least one compound of formulae Ea toEd

in which R⁰ is H, n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each havingup to 9 carbon atoms.
 7. The medium according to claim 1, comprising atleast one compound of formulae IIa to IIg

in which R⁰ is H, n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each havingup to 9 carbon atoms.
 8. The liquid-crystalline medium according toclaim 1, comprising at least one compound of following formulae:

in which R⁰ is n-alkyl, oxaalkyl, fluoroalkyl, alkenyloxy or alkenyl,each having up to 9 carbon atoms, Y¹ is H or F, alkyl and alkyl* areeach, independently a straight-chain or branched alkyl radical having1–9 carbon atoms, alkenyl and alkenyl* are each, independently astraight-chain or branched alkenyl radical having up to 9 carbon atoms.9. The liquid-crystalline medium according to claim 1, characterised inthat the proportion of compounds of the formula IA in the mixture as awhole is 5 to 40% by weight.
 10. An electro-optical liquid-crystaldisplay containing a liquid-crystalline medium according to claim
 1. 11.A liquid-crystalline medium based on a mixture of polar compounds ofpositive dielectric anisotropy, comprising at least one ester compoundof formula I

and at least one compound of formula IA

wherein: R¹ and R² are each, independently H, a halogenatedunsubstituted alkyl radical having from 1 to 15 carbon atoms, where oneor more CH₂ groups in these radicals is optionally independentlyreplaced by —C≡C—, —CH═CH—, —O—, —CO—O— or —O—CO— in such a way that Oatoms are not linked directly to one another, X¹ and X² are each,independently F, Cl, CN, SF₅, SCN, NCS, a halogenated alkyl radical, ahalogenated alkenyl radical, a halogenated alkoxy radical or ahalogenated alkenyloxy radical, each having up to 6 carbon atoms, Z¹ andZ² are each, independently —CF₂O—, —OCF₂— or a single bond, where Z¹≠Z²,

and are each, independently

L¹ to L⁶ are each, independently H or F, further comprising at least onecompound of following formulae:

in which R⁰ is n-alkyl, oxaalkyl, fluoroalkyl, alkenyloxy or alkenyl,each having up to 9 carbon atoms, Y¹ is H or F, alkyl and alkyl* areeach, independently a straight-chain or branched alkyl radical having1–9 carbon atoms, alkenyl and alkenyl* are each, independently astraight-chain or branched alkenyl radical having up to 9 carbon atoms.12. A liquid-crystalline medium based on a mixture of polar compounds ofpositive dielectric anisotropy, comprising at least one ester compoundof formula I

and at least one compound of formula IA

wherein: R¹ and R² are each, independently H, a halogenatedunsubstituted alkyl radical having from 1 to 15 carbon atoms, where oneor more CH₂ groups in these radicals is optionally independentlyreplaced by —C≡C—, —CH═CH—, —O—, —CO—O— or —O—CO— in such a way that Oatoms are not linked directly to one another, X¹ and X² are each,independently F, Cl, CN, SF₅, SCN, NCS, a halogenated alkyl radical, ahalogenated alkenyl radical, a halogenated alkoxy radical or ahalogenated alkenyloxy radical, each having up to 6 carbon atoms, Z¹ andZ² are each, independently —CF₂O—, —OCF₂— or a single bond, where Z¹≠Z²,

and are each, independently

L1⁻⁶ are each, independently H or F. With the proviso that at least oneof the rings A and B is